The prices are more than I'm willing to pay, but there are a bunch of mechanical keyboard whackos out there inflating the prices for this kind of stuff. The market will do what the market does, whether the motives of the participants is in any way rational.
I also suspect that there is a large element of supply & demand at work: for lots of reasons this kind of equipment is becoming rarer and rarer, and that's simply got to drive the price up if there is any kind of demand. The prices would certainly have been a lot lower a decade or two ago, but those days are gone. You might also be able to find some of these things at a hamfest for lower prices, if you can find a hamfest to go to (I haven't seen a hamfest in my area in many years), but even there you are likely to see increasing scarcity of older equipment.
Finally, my impression is that IBM equipment generally didn't make its way into private hands, both because these systems were marketed almost exclusively to corporate and institutional buyers, and because IBM preferred to lease equipment rather than sell it outright. This had the effect of radically restricting the aftermarket on such things, which is reflected in current scarcities. You'll have similar difficulty finding systems from other manufacturers in the same space (e.g. Burroughs, Honeywell, Univac, Sperry, etc.).
